Ultrasound-Guided Forearm Block

Ultrasound imaging of individual nerves in the distal upper limb allows for reliable nerve blockade. The two main indications for these blocks are a stand-alone technique for hand and/or wrist surgery and as a means of rescuing or supplementing a patchy or failed proximal brachial plexus block. The main advantages of the ultrasound-guided technique over the surface-based or nerve stimulator-based techniques are the avoidance of unnecessary proximal motor and sensory blockade, that is, greater specificity. Additional advantages are avoidance of the risk of vascular puncture and a reduction in the overall volume of local anesthetic used. There are a variety of locations where a practitioner could approach each of these nerves, most of which are similar in efficacy. In this chapter, we present the approach for each nerve that we favor in our practice.

Radial Nerve

The radial nerve is best visualized above the lateral aspect of the elbow, lying in the fascia between the brachioradialis and the brachialis muscles (Figure 2). The transducer is placed transversely on the anterolateral aspect of the distal arm, 3-4 cm above the elbow crease (Figure 1A). The nerve appears as a hyperechoic, triangular, or oval structure with the characteristic stippled appearance of a distal peripheral nerve. The nerve divides just above the elbow crease into superficial (sensory) and deep (motor) branches. These smaller divisions of the radial nerve are more challenging to identify in the forearm; therefore, a single injection above the elbow is favored because it ensures blockade of both. The transducer can be slid up and down the axis of the arm to better appreciate the nerve within the musculature surrounding it. As the transducer is moved proximally, the nerve will be seen to travel posteriorly and closer to the humerus, to lie deep to the triceps muscles in the spiral groove (Figure 3)

Figure 3: Sonoanatomy of the radial nerve in the spiral groove of the humerus. RN, radial nerve.

The median nerve is easily imaged in the midforearm, between the flexor digitorum superficialis and flexor digitorum profundus, where the nerve typically appears as a round or oval hyperechoic structure (Figure 4A and B). The transducer is placed on the volar aspect of the arm in the transverse orientation and tilted back and forth until the nerve is identified (Figure 1B). The nerve is located in the midline of the forearm, 1-2 cm medial and deep to the pulsating radial artery. The course of the median nerve can be traced with the transducer up and down the forearm, but as it approaches the elbow or the wrist, its differentiation from adjacent tendons and connective tissue becomes more challenging.

The ulnar nerve can be easily imaged in the midforearm, immediately medial to the ulnar artery, which acts as a useful landmark. Similar to the radial and median nerves, the ulnar nerve appears as a hyperechoic stippled structure, with a triangular to oval shape (Figure 5A and B). The ulnar artery and nerve separate, when the transducer is slid more proximally on the forearm, with the artery taking a more lateral and deeper course. The ulnar nerve can be traced easily proximally toward the ulnar notch, when desired, and the level of the blockade can be decided based on the desired distribution of the anesthesia as well as the ease of imaging and accessing the nerve. Sliding the transducer distally shows the nerve and artery becoming progressively shallower together as they approach the wrist where the ulnar nerve lies medial to the artery.

As is the case with the landmark-based distal blocks, anesthetizing the radial, median, and/or ulnar nerves provides sensory anesthesia and analgesia to the respective territories of the hand and wrist. Note that the lateral cutaneous nerve of the forearm (a branch of the musculocutaneous nerve) supplies the lateral aspect of the forearm, and it may need to be blocked separately by a subcutaneous wheal distal to the elbow if lateral wrist surgery is planned.

Equipment

Equipment needed includes the following:

Ultrasound machine with linear transducer (8-14 MHz), sterile sleeve, and gel

Standard nerve block tray

One 20-mL syringe containing local anesthetic

A 2-in, 22-25 gauge short-bevel insulated stimulating needle

Peripheral nerve stimulator

Sterile gloves

Any patient position that allows for comfortable placement of the ultrasound transducer and needle advancement is appropriate. Typically, the block is performed with the patient in the supine position. For the radial nerve block, the arm is flexed at the elbow and the hand is placed on the patient's abdomen (Figure 1A). This position allows for the most practical application of the transducer. The median and ulnar nerves are blocked with the arm abducted and placed on an armboard, palm facing up. (Figures 1 B,C)

Figure 6: (A) Needle position to block the radial nerve (RN) at the elbow. BM - Brachialis Muscle, BrM - Brachioradialis muscle. (B) Local anesthetic (area shaded in blue) distribution to block the RN above the elbow. (1) Biceps brachii muscle. 

Radial Nerve

With the patient in the proper position, the skin is disinfected and the transducer positioned so as to identify the radial nerve. The needle is inserted in-plane, with the goal of traversing the biceps brachii muscle and placing the tip next to the radial nerve (Figure 6A). If nerve stimulation is used, a wrist or finger extension response should be elicited when the needle is in proximity to the nerve. After negative aspiration, 4-5 mL of local anesthetic is injected (Figure 6B). If the spread is inadequate, slight adjustments can be made and a further 2-3 mL of local anesthetic administered.

With the arm abducted and the palm up, the skin of the volar forearm is disinfected and the transducer positioned transversely on the midforearm. The median nerve should be identified between the previously mentioned muscle layers. If it is not immediately visualized, the transducer should be positioned slightly more laterally and the radial artery identified, using color Doppler ultrasound. Sliding back to the midline, the nerve can be seen approximately 1-2 cm medial and 1 cm deep to the radial artery. The needle is inserted in- plane from either side of the transducer (Figure 7A). After negative aspiration, 4-5 mL of local anesthetic is injected (Figure 7B). If the spread is inadequate, slight adjustments can be made and a further 2-3 mL of local anesthetic administered.

Figure 7: (A) Needle (1) position for the block of the median nerve (MN) at the forearm. (B) Distribution of local anesthetic for block of the MN at the forearm.

Then the transducer is positioned more medially until the ulnar nerve is identified. The use of color Doppler ultrasound can aid in finding the ulnar artery, which always lies lateral to the nerve at this level. The nerve should then be traced up until the artery "splits off," to minimize the likelihood of arterial puncture. The needle is inserted in-plane from either side of the transducer (the lateral side is often more ergonomic) (Figure 8A). After negative aspiration, 4-5 mL of local anesthetic is injected (Figure 8B). If the spread of the local anesthetic is inadequate, slight adjustments can be made and a further 2-3 mL administered.

The use of a tourniquet, either on the arm or forearm, usually requires sedation and/or additional analgesia.

Figure 8: (A) Needle (1) position for the block of ulnar nerve (UN) at the forearm. (B) Distribution of local anesthetic (area shaded in blue) for the block of the UN at the forearm.